In the treatment of water, wastewater and other liquids, filters of various types are commonly used to filter out solid particles. Media filters often make use of filter media formed by a mixture of fine sand and anthracite. The liquid passes through the filter media in order to remove some of the impurities.
The drain system for this type of filter must be constructed to allow the filtered liquid to drain away while avoiding carrying away the media particles with the liquid. The particles typically have a size of about 0.020 inch or even less, so preventing them from being washed away with the liquid can be difficult. The media must be backwashed periodically in order to dislodge materials that plug the media and unduly impede the liquid flow.
In the past, it has been the prevalent practice to provide underdrain systems that include a concrete slab equipped with nozzles having small slits, clay tiles with small holes, or plastic blocks with a permeable cap. Although these types of underdrains can function in a satisfactory manner in many applications, they are not wholly without problems. The need to provide a concrete slab with nozzles results in considerable construction expense, clay tiles often break and plug, while plastic blocks are also expensive.